The present invention relates to improved dewatering of a paper web in a press section of a papermaking machine. More particularly, the invention relates to press felts that incorporate or are arranged to provide a reduced or retarded spring back elasticity after the mid nip point between the press rolls to reduce rewetting of the paper web.
Press felts are endless belts which may contain a seam and which are used to convey an embryonic paper web from the forming section, through the press and into the dryer section of a papermaking machine so as to dewater and ultimately dry the paper product so that it is suitable for use. In the press section, at least one press nip is typically provided between either a pair of rotating cylindrical rollers, or a roller and concave shoe. The embryonic paper web passes through the at least one press nip laid either upon a single felt, or sandwiched between at least two press felts. As the web passes through the at least one press nip, water is expressed from it and passes into the at least one press felt.
Papermaker's press felts are well known. See, for example, U.S. Pat. No. 4,199,401 to Liu et al., U.S. Pat. No. 4,356,225 to Dufour, U.S. Pat. No. 4,414,263 to Miller et al., U.S. Pat. No. 4,806,413 to Penven , U.S. Pat. No. 5,360,656 to Rexfelt et al., and U.S. Pat. No. 5,864,931 to Best et al. These felts are usually comprised of a woven base fabric (typically formed of nylon or similar polymeric yarns) to which is attached, generally by needling, at least one layer of a pre-tacked staple fiber web, commonly referred to as a batt. Typical press felt batts will usually include between one and about 5 or more layers of a pre-tacked staple fiber web needled onto a first planar surface of the base fabric (usually the surface which, when in use, will be in contact with the paper sheet, and is hereafter referred to as the “PS”) to form a PS batt, and from none to one or more layers needled to the opposite planar surface (which when in use will be in contact with the equipment of the paper machine, and is hereafter referred to as the “MS”) to form the MS batt. The staple fibers used to form either or both the MS and PS batt are typically made from one or more nylons, polyesters or other polymeric materials such as are commonly employed in the manufacture of industrial textiles.
The batt provides a smooth surface for the paper web and a void volume into which water, which has been expressed from the paper web at the press nip, can be received. The base fabric provides some additional void volume, as well as a stable structure to which the batt can be attached. The base fabric is typically comprised of interwoven polymeric monofilament or multifilament yarns to which the batt is attached, generally by needling or other entanglement process such as is known in the art.
After the paper web has been pressed in at least one nip in the press section, it will still contain an appreciable amount of water, as much as from 30% to about 60% or more by weight. This remaining water must now be removed in the dryer section of the papermaking machine in order to provide a paper product. The final drying of the paper product is typically carried out by evaporative means, which requires a large amount of energy. This adds substantially to the cost of manufacturing the paper product. Generally, a 1% increase in the dryness of the sheet exiting the press section will translate into about a 4% energy savings in the dryer section. It is also possible that the speed of the paper machine may have to be reduced or at least limited due to the evaporative capacity of the dryer section.
The generally accepted theory of operation of a papermaking machine press section is described in Wahlstrom, P. B., A Long Term Study of Water Removal and Moisture Distribution On A Newsprint Machine Press Section, Parts I and II, Pulp and Paper Mag. Can., 60, No. 8: T379–T401 (August 1960); Id., No. 9: T418–T451 (September 1960). Briefly, on the entrance side of the nip, water is removed by compression and the resulting hydraulic pressure to a maximum dryness, which is achieved at the mid nip point. However, rewetting of the paper web occurs through capillary action that draws water back from the press felt once the paper web has passed through the nip.
There have been a number of attempts to reduce this rewetting, such as by using hydrophilic or hydrophobic fibers or coatings in or on press felts, or the introduction of an anti-rewetting layer on or between the mid layers of the press felt batt. However, while such measures have resulted in limited improvements, there is still the need for improved dewatering in the press section to reduce production costs.
Thus, it would be highly desirable if the water removal in the press section could be improved to increase the amount of water transported away from the paper product as it passes through the press section.